HD 2600 Pro weird blackouts

OK - running Vista Home Premium, and using MCE to act as a PVR via a freeview tuner.

Since changing the 8500GT with a 1600 Pro, when watching recorded TV, I'm seeing strange "pauses" where the screen goes black. Audio continues fine, and then after a 1 or 2 second pause, the picture resumes where it left off.

It happens about once every five minutes, so not unbearable but a little irritating. It certainly didn't do it with the 8500. Running latest 32 bit drivers from the web, rather than from the CD.

Anyone experienced this, or have any ideas?